Two different systems may be required to operate in synchronism with each other to provide two different types of information relative to a single objective. For example, a radar system and an IFF (Identification Friend or Foe) system may be coordinated to provide different types of information with respect to a common target. In such an arrangement, both systems may be rendered synchronously operative by being triggered from a common signal such as the radar trigger signal, for example.
Where two such systems are so coordinated, the radar return signal will provide azimuth, elevation, and range information with respect to a target and the IFF return signal will provide identification of the same target. In its operation, however, the IFF signal frequently is required to undergo encoding operations and in many instances cryptographic procedures must also be completed. These requirements for encoding and/or cryptographic procedures introduce delay into the IFF signal which is not present in the associated radar signal.
It can be readily understood, however, that it is desirable for signals which pertain to the same target to be coordinated and synchronized in time for purposes such as their simultaneous display as may be accomplished in a conventional PPI visual presentation, for example.
Accordingly, it is highly desirable that an appropriate accurate and reliable correction be made to the IFF signal which is delayed by reason of encoding or cryptographic procedures. When the common trigger signal, such as the trigger of the radar system, has a constant pulse repetition frequency, a known amount of such undesired delay that may be due to encoding or cryptographic procedures can be compensated for by providing a "pre-trigger" signal. In the use of such a "pre-trigger" signal the IFF signal is radiated before the radar signal by an amount of time equal to the known delay due to encoding or cryptographic procedures so that the radar return signal and the IFF return signal are synchronized in time upon their return from the same target and the time elapse relative to the initial trigger is the same for both signals, representing range.
In the prior art, two coordinated systems such as radar and IFF systems operating in conjunction with each other have been provided with a selectively variable pre-trigger arrangement which may be used to pre-select and pre-set the amount by which a pre-trigger signal, such as is used to actuate an IFF radiation, for example, will precede the transmitted radar signal. Such selectively preestablished amount of pre-triggering compensates for the delay which may occur by encoding and/or cryptographic procedures at the target before the target, in its transponding function, returns the IFF signal to its origin.
However, as may be readily understood by those skilled and knowledgeable in the pertinent arts, where the trigger signal varies in its pulse repetition frequency, conventional techniques for generating a properly timed pre-trigger signal cannot be employed, since the pulse repetition interval between successive trigger signals will vary. Therefore, the prior art method of generating a pre-trigger signal a pre-calculated amount of time after each preceding trigger signal will not provide the proper desired synchronized time relationships to accurately, reliably, and uniformly compensate for a known or determinable time delay introduced in an IFF signal, for example, by reason of encoding or cryptographic procedures which may take place at the target before the return of the IFF signal.
Thus, there is a need for a system which is capable of retiming a delayed signal relative to another signal so as to restore their original time relationship as derived from a common trigger signal, where such trigger signal may vary in pulse repetition frequency.